The continuing purpose of this research is to establish failure criteria for the articulating surfaces of polyethylene (UHMWPE) components used in total joint replacement systems and to apply these criteria to optimize implant design. Observations of retrieved components revealed distinct damage modes apparently caused by fatigue fracture mechanisms. To date, the design decisions for UHMWPE components have been based on the assumption that the particular stresses and stress distribution (namely, the maximum shear stress and the range of maximum principal stress) discovered in our analytical models are indeed those responsible for causing these damage modes. To verify the direct relationship between specific stress states and the production of surface damage, the conditions under which growing fatigue cracks in UHMWPE will change direction must be established which will cause small defects on and below the surface to propagate and create the observed damage modes. The approach is based upon principles of fracture mechanics. Fatigue crack propagation experiments will be conducted on standardized UHMWPE specimens to determine the relationship between the cyclic stress intensity and the crack growth rate and direction. Test variables will be the angle of inclination of the crack relative to the direction of the applied loading and the state of preconditioning of the material under uniaxial cyclic loading prior to testing. Tests will be conducted on specimens made from both conventional UHMWPE and enhanced forms of UHMWPE. The empirical relationship will be used as input to a numerical method for predicting fatigue crack propagation to demonstrate that the method correctly predicts fatigue crack propagation in UHMWPE. This will be accomplished by modelling the test specimen geometry and loading conditions from the fatigue tests and comparing the computed crack propagation rates and directions with those measured experimentally. If fatigue crack propagation in UHMWPE cannot be described on the basis of linear elastic fracture mechanics, the analytical method will be modified to include non-linear material behavior around the crack tip.